Damir Lacković

Evolution of the Veternica cave (Medvednica Mountain, Croatia) drainage system: insights from the distribution and dating of cave deposits

Detailed field examination, U-Th age dating, and stable isotope analysis of Veternica flowstone and shelfstone deposits have been employed in order to unravel details about the geological history of Veternica Cave (Medvednica Mountain, Croatia). The study was carried out in the hydrologically inactive part of the main cave channel, which is developed mainly along the unconformity between Triassic dolostone and Miocene limestone. For 180 m from the cave entrance, (located at 320 metres above sea level (asl)), the morphology of the main channel reflects exclusively phreatic conditions in the cave until the end of its hydrological activity. From 180 to 390 m, the phreatic channel has a secondary vadose entrenchment in the bottom part as marked by massive flowstones at elevations from 306 to 313.5 m asl. From 390 m farther inside the cave (in the upstream direction), the main channel has a tall, narrow cross-section and is of mainly vadose origin. In this part of the cave shelfstone precipitates are observed at 9 different levels ranging from 318.8 to 320.2 m asl, indicating the water palaeolevels in the cave. U-Th dating revealed the age of the highest shelfstone (320.2 m asl) of ~380 kyrs BP, and the age of the lowermost analyzed shelfstone (318.9 m asl) of ~245 kyrs BP. Dating of flowstone deposits, located below the shelfstone level, revealed their formation from ~235 to 205 kyrs BP. A relatively rapid water table lowering and transition from phreatic to vadose conditions occurred within the cave, from ~245 to 235 ka BP, between formation of the youngest shelfstone (representing phreatic conditions below their level) and the oldest flowstone (marking the beginning of vadose conditions). The results provide unique new information about Veternica Cave genesis and geomorphologic evolution of the Medvednica Mountain area.

Hydrologic Evolution of a Carbonate Aquifer (Dinaric Karst, Croatia)

Dinaric Karst terrains contain specific, lam inatedspeleothems, which line all surfaces of subterraneanvoids including Cretaceous bedrock limestone, and oldervadose speleothems and clastic sediments. The speleotherms may attain a thickness of one metre. Deposition of these speleothems took place during a longterm phreatic period in the Late Quaternary. This is incont rast to well-known, widespread speleothems, whichwere, and are deposited in vadose and uppermost phreaticsettings. Furthermore, the origin of these phreaticspeleothems departs from common experience by the dominant dissolutional widening of underground conduits when saturated. Phreatic speleothems can be importantstrat igraphic marker for the stratigraphy of cavesediments. This approach revealed a sequence of threehydrologic stages: (1) prephreatic stage with dominantvadose conditions and dissolution processes, (2) phreaticstage when the ground-water level was very high inthe large karst area, and when the volume of the voidsdecreased, and (3) the last, dominantly vadose stageduring which voids were enlarged for the second time,and phreatic speleothems may be covered by youngerclastics and vadose speleothems.